opaque 2 Modifiers Act Post - Transcriptionally and in a Polar Manner on y - Zein Gene Expression in Maize Endosperm

The opaque2 (02) modifier genes convert the soft endosperm of an 02 mutant to a hard, vitreous phenotype. The primary biochemical change associated with the expression of these genes is a twoto threefold increase in synthesis of the 27-kD y-zein storage protein. To investigate the mechanism of modifier gene activity, we examined the level of y-zein mRNA and protein synthesis during the early stages of endosperm development in normal, 02, and modified 02 genotypes. Although the 02 mutation was found to reduce expression of the 27-kD y-zein genes, the activity of 02 modifier genes dramatically increased the level of both y-zein protein and mRNAs as early as 16 days after pollination. At this stage, transcription of y-zein genes is reduced by -50% in both 02 and modified 02 genotypes compared to wild type. Thus, it appears that the modifiers regulate y-zein synthesis through a post-transcriptional mechanism. Analysis of transcripts from the two nearly identical genes (A and B) encoding the 27-kD y-zein protein showed differences in the mRNA ratios in different genotypes. In modified 02 mutants, accumulation of A over B transcript was greatly enhanced during endosperm development. Somatic recombination at this locus was found to reduce the number of B genes in the endosperm, but this could not account for the preferential accumulation of the A transcript. Our results suggest that a product of the 02 modifier genes increases the translation or stability of the A gene mRNA, leading to enhanced synthesis of 27-kD y-zein protein.